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Tester mentioned a month or so ago that the ratio of rim inner width to actual tire width should be between 0.5 to 0.75, at least for slicks.
When I asked Syntace about tire options for their 30mm rims (24.5mm internal width) they said 33mm was perfectly safe, and pointed out that using a relatively wider rim means more air volume in the tire, which increases stability.
Most of the new aero rims from zipp, Hed, enve etc. seem to be intended for use with tires that match the external width of the rims, so that air flow isn't interrupted at it passes from tire to rim or vice versa. Not so relevant for functional bikes but that seems to be the way things are headed.
Rudder2-
I have to agree with you on the slicks! But if you're going up to 0.75 you really need a rim with an aggressive hook like a Zipp or Enve rim.
I wouldn't recommend going up to 0.75 with road tubeless rims (or your Syntace 30mm rims). Sure you'll get them on there with your shop pump/air compressor and some love. And the ride will be amazing. But if you have a puncture and try to get going again by putting in an inner tube and inflating it with a small jerseypocket pump you'll have a really hard time getting the bead seated again (close to impossible). I've seen this go wrong a lot of times!
Jaap
what do you mean by rim width? outer, inner or bead seat?
If you meant inner, I agree but would maybe stretch it to 60% - 65%